Current mirror circuits are used in a wide range of electronic circuits where a single reference current source is used to control the output of one or more “mirrored” current sources. In one common configuration, a reference or “bias” current source generates an electrical current at a predetermined level. The bias current source is connected to a bias transistor. The bias current source generates a predetermined reference current and the bias transistor also passes the current at the same level as the bias current source, which affects a voltage level at the gate of the bias transistor. The gate of the bias transistor is connected to the gates of one or more additional or “mirrored” transistors that also pass current from an external power supply with reference to the gate voltage from the bias transistor. The mirrored current sources often produce current at output levels that are a multiple of the reference current source. For example, some current mirror configurations generate an output current at the same magnitude as the reference current source (e.g. a multiplier of one). In other embodiments, the mirrored output current is an integer multiple (e.g. a multiplier of 2×, 3×, 4×, etc.) or non-integer multiple (e.g. 0.5×, 1.5×, 2.5×, etc.) of the reference current. A single reference current source can also be mirrored by an array of multiple current outputs that each generate an output current based on the single reference current source.
In some configurations, the output of the current mirror circuit is used in a larger circuit that processes signals at a particular frequency. For example, digital to analog converter (DAC) circuits often receive a digital input signal that is generated at a predetermined frequency and generate analog output signals corresponding to the value of the analog signal. The current mirror circuit in a DAC includes one or more current sources that are selectively activated and summed together to produce an analog output signal with reference to the digital input signal. While DACs are one example of electronic components that employ current mirrors, the current mirror circuits are used in other circuit configurations as well.
One issue with operation of a current mirror is that the output signal includes noise from several different sources. One source of noise in a current mirror comes from a biasing circuit that typically includes a transistor that is operatively connected to the reference current source. A voltage at the gate of a bias transistor is influenced by the flow of current through the reference current source. The gate of the bias transistor is electrically connected to the gates of one or more additional transistors in the current mirror circuit to control the gate voltage levels and corresponding levels of current that flow through the additional transistors.
Prior art solutions to reduce the impact of noise in the output signal include either increasing the magnitude of the reference current through the bias transistor to reduce the relative level of the bias noise compared to the overall level of current or adding a capacitor between the biasing transistor and the transistors that produce the mirrored current to form a filter. However, increasing the current level through the bias transistor also increases the overall power consumption of the current mirror circuit. Additionally, capacitors that are large enough to be effective at filtering noise in many circuits are too large to be incorporated in the current mirror in a practical manner. In many applications, only a comparatively narrow frequency band is of interest to the operation of the circuit that employs the current mirror. For example, in a DAC that is connected to a microelectromechanical system (MEMS) gyroscopic sensor, the frequency of interest corresponds to a narrow range of frequencies around a frequency of oscillation of the sensor. For example, the frequency of oscillation in many MEMS gyroscopes is typically in a range of tens or hundreds of kilohertz, with a frequency band of interest in a range of tens or hundreds of hertz (e.g. an 80 Hz frequency band of interest around a 25 kHz oscillation frequency). Consequently, improvements to current mirror circuits that attenuate noise in a predetermined frequency range without requiring large capacitors for filters would be beneficial.